1. Field of the Invention
This invention relates to up conversion mixers as used in transmitters, to methods of producing mixed signals, and to mixed signals produced by such apparatus or methods.
2. Discussion of the Related Art
A mixer performs a multiplication of two input signals (A at frequency fA and B at fB) resulting in an output signal C at fA+/−fB. Depending on the frequency nature of the input and output signals, up-conversion and down-conversion mixers can be distinguished. In a down-conversion structure two high frequency signals (Local Oscillator LO and Radio Frequency RF) are the input signals resulting in a low frequency (Intermediate Frequency IF or Low Frequency LF) output. This is used in receiver structures. In an up-conversion mixer, as used in transmitters, the output is the high frequency RF signal and the low frequency LF is an input. The up-conversion of the baseband signal has to be performed linearly. A non-linear conversion will result in unwanted frequency spurs and spectral regrowth. A notable indication of linearity is the level of unwanted third order baseband products (LO.3BB) produced by the mixer from multitone inputs.
When the linearity of a transmitter is discussed, two different aspects have to be considered. A first aspect is the “in-band linearity”. When a local oscillator signal is mixed with a single-tone baseband (BB) signal, this results in a mixing product LO.BB with frequency components at f{LO}+/−f{BB}. However, because of the non-linearity of the mixers and/or because of intermodulation products, also components at f{LO}+/−x.f{BB} are formed. This non-linearity is very important because these unwanted mixing components can not be filtered out.
When a spectrum is applied instead of a single tone, this kind of non-linearity causes regrowth of the spectrum which can cause a serious bit-error-rate degradation in the adjacent channels at the receiver side. The relation between the single tone behaviour and the spectral regrowth is dependent on the input signal modulation type and can be mathematically calculated.
A second important linearity specification is the “harmonic distortion around multiples of the LO frequency”. These components can be filtered out after a single-channel transmitter, and so are less of a concern for the present purposes.
Prior art solutions include similar structures for up- and down-conversion mixers based on variants on the classical (bipolar) Gilbert cell topology. The mixer implementations are not intrinsically linear. It is useful to distinguish between two stages, the first being generating the linear baseband current and the second being frequency translation of this input signal to RF. FIG. 1 shows an example of the known Gilbert cell having baseband and the RF functional subcircuits (10, 20). These are coupled in series in a path between a pair of power supply lines. The issue of linearity of the baseband current is essentially a basic low frequency analog issue, and can be dealt with in various ways by experienced analog designers, following established practice. Many variants (e.g. two stage arrangements) exist for the baseband part. The Gilbert cell topology was developed for bipolar implementation. More recently CMOS versions have been implemented. For these, proper switching of the modulating transistors requires a large LO voltage swing. Considering the limited frequency ability of the technology, this large high frequency (GHz range) LO signal conflicts with power consumption requirements.
The result of this is that the output signal's linearity is limited. When applying a sinewave baseband input, this non-linearity is seen as spurious components at f(LO)+/−#.f(BB). The linearity specification can be improved by reducing the baseband input signal amplitude, but this is not favorable from power consumption point of view: reducing the baseband signal will proportionally reduce the amount of RF output signal and amplifying a GHz signal linearly is very power-hungry. Therefore there is a need for a solution having good linearity in case of large baseband input values and/or relatively small LO signals. Existing linearization techniques like resistive degeneration intend to provide a linear baseband current to the modulating element. However even with an ideally linear baseband input, harmonic components of the baseband signal will appear in the output spectrum.
An example of a CMOS Gilbert cell with resistive degeneration is shown in U.S. Pat. No. 6,433,647. A low-noise quadrature phase I-Q modulator has a pair of Gilbert cell input stages. The degenerative resistor is located between the source terminals of the two transistors of the baseband part of the cell.
U.S. Pat. No. 5,095,290 discloses a modulator that uses negative feedback to improve the linearity of a Gilbert Cell. As with other types of balanced mixers, a Gilbert Cell lacks an output signal which is suitable for developing an appropriate feedback signal. The '290 patent overcomes this problem by using a first and a second Gilbert Cell. A first half of each Gilbert Cell is used to provide the modulated output signal, and a second half of each Gilbert Cell is used to reconstruct the modulating signal from a temporally sliced signal that is generated by each Gilbert Cell. The first Gilbert Cell provides a temporally sliced waveform that is complimentary to the temporally sliced waveform provided by the second Gilbert Cell. When these two temporally sliced waveforms are combined, they produce a usable modulator feedback signal. However, although the temporally-sliced waveforms should theoretically fit together in a complementary manner, in practice this requires a very high degree of matching between the active devices utilized in the Gilbert Cells.
Another example of a CMOS Gilbert cell arrangement is shown in U.S. Pat. No. 6,242,963 relating to a differential mixer with improved linearity. This explains that two types of loads have commonly been used for mixers based on Gilbert cells: resistive loads and MOSFET loads. In systems based on resistive loads, generally the gain of the mixer can only be increased by increasing the value of the resistors. Resistors with large values have higher parasitic capacitance associated with them and therefore can dramatically reduce the mixer speed and bandwidth. At the same time, large resistive loads combined with the relatively high bias currents required for high-speed operation can cause problems in the biasing of the Gilbert cell transistors and thereby effectively impose practical limits on the signal swing of the mixer output. Furthermore, the size of the resistive load may vary due to process variations by 30-40%, a variation that can change the gain substantially. In systems based on MOSFET loads, many of the difficulties associated with resistive loads no longer constrain the design. However, the nonlinear voltage-current characteristics of MOSFETs can create signal harmonics at the output nodes so that the output of the mixer is degraded. Accordingly, the patent suggests using a load containing transistors that are configured as a diode and a triode, where these circuits are additively combined to achieve substantially linear voltage-current characteristics over a predetermined range. By using substantially linear loads instead of conventional single-transistor loads, the linearity of the mixer can be improved. This effectively shows how a current generated in the mixer can be converted linearly into a voltage. However this does not help if the current generated is non linear, as the linear current to voltage conversion will not improve the linearity of the generated current.
In U.S. Pat. No. 6,404,263, there is a bipolar transistor Gilbert mixer for a wireless communications system having a differential amplifier (baseband part) that translates an input intermediate frequency voltage signal or an input radio frequency voltage signal to current signals that are supplied to a doubly-balanced switching modulator (RF part). This develops a differential mixed output radio frequency signal or intermediate frequency signal that is the product of the current signals and a local oscillator signal. Included at the output of the differential amplifier are reactance circuits or capacitors each of which provides a low impedance to ground at the second harmonic of the local oscillator signal and a high impedance at the frequency of the input radio frequency signal or input intermediate frequency signal. This counteracts the possibility of high second harmonic contents in the emitters of the transistors of the RF part when the input level of the local oscillator signal is high. The second harmonic is of concern if it has the greatest magnitude when compared to the other harmonics, in degrading the conversion gain and degrading the linearity of the mixer. However such harmonics of the LO are out of band and so can be countered by filtering. In many cases other components are more important to the overall linearity. The problem originates from the fact that with high LO input, significant even harmonics (mainly 2nd) are common mode at the LO base input. This common-mode input will reflect in this same frequency component at the emitter of the LO transistor. In this way, the effective voltage between base and emitter is reduced resulting in a reduced switching. This is solved by filtering the 2*f(LO) component. A resonance circuit that delivers a low impedance at 2LO. This only covers a small frequency band (e.g. not around 3LO). Only the 2LO component is trageted, as this is sufficient for this problem.
U.S. Patent Application 20010021645 shows a CMOS implemented Gilbert cell for a down-conversion mixer. A source of each of the differential pairs of the RF part and a source of the differential amplifier of the baseband is coupled to ground through an impedance circuit. This has high impedance for operational frequency of the circuit and short-circuited for D.C. current. This allows a bias potential to a gate of a transistor of a differential pair to be independent of a bias potential of a gate of a transistor of a differential amplifier. An independent bias potential allows a differential pair and a differential amplifier to have optimized bias potential so that a phase error of an output IF signal can be minimized. The document also shows a load (ZL1) coupled from the drain terminals of the transistors of the baseband part to the power supply potential VDD. This is apparently a consequence of providing independent biasing. As the circuit is for a down conversion mixer, the load would need to be other than capacitive, for the circuit to work. There is no suggestion that this load can be used for other purposes.